basedon the large negative value of the tau statistic we reject the null hypothesis that deltapce has a unit root and accept the alternative that it is stationary. collecting the results from the unit root tests on pce and deltapce , we can say that the series pce is I (1). had the null hypothesis of a unit root not been rejected in (16.4.11) , we would have concluded that pcf is I (2) or integrated of an order higher than 2.